Just by chance - double check that the location in your profile is really really correct. The other day, another member was struggling with incorrect timestamps - it turned out that her profile was set to a Melbourne in the US instead of the correct Australian Melbourne. Maybe it's yet something different but it might be worth looking into

Thank you for your answer but I specified my city (Paris, France) well and the family plan I want to join is also in France...

I lived in another country two years ago so I guess the issue may come from here. I hope Strava will answer me quickly!